Chest wall deformities are generally divided into pectus excavatum, also called funnel chest, and pectus carinatum, also called pigeon chest.
As illustrated in FIG. 1, the rib cage includes ribs 10, a sternum 12, rib cartilages 14, and thoracic vertebrae 16, which are connected to each other.
In the rib cage, pectus excavatum and pectus carinatum do not refer to deformities in the ribs 10 but refer to deformities in the rib cartilages 14 interconnecting the ribs 10 and the sternum 12 positioned at the center of the chest.
It is reported that such deformities are not severe at birth but develop with age.
Particularly, in the case of pectus excavatum, a sunken chest wall presses the heart or the lungs and can cause functional disorders in the pressed part.
The Ravitch procedure and the Nuss procedure are known surgical treatments for pectus excavatum.
The Ravitch procedure involves making a large incision in the front chest and completely removing an abnormal rib cartilage through the incision. After surgery, the chest wall may be weakened, may lose soft motion functionality thereof due to adhesion, and may have a large scar thereon.
The Nuss procedure was invented by Dr. Donald Nuss (USA) in 1997 and is a procedure capable of replacing the Ravitch procedure to treat pectus excavatum.
In the Nuss procedure, for example, when a part of the chest wall indicated by hatched lines H in FIG. 2a is sunken, incisions of about 1 to 2 cm are made in the armpits, and a curved pectus bar 18 is inserted through the incisions as illustrated in FIGS. 2a and 2b. 
Then, the curved pectus bar 18 is flipped in a direction indicated by an arrow A in FIGS. 2a and 2b, to push the sunken sternum 12 and the rib cartilage 14 outward as illustrated in FIG. 2c. 
Subsequently, two ends of the pectus bar 18 are supported by the ribs 10 corresponding thereto until the chest is corrected to a normal chest wall shape.
As described above, compared to the Ravitch procedure, since surgical scars of only 1 to 2 cm are created at two sides of the chest and the chest is corrected to a normal chest wall shape without resecting the rib cartilage 14, the Nuss procedure may not only constantly maintain flexibility and resilience of the chest but also have a short operation time and a small amount of blood loss during operation.
In the Nuss procedure, the pectus bar 18 should be stably supported against restoring forces of the sternum and the rib cartilage 14, which tend to return to original states thereof before correction, until the sternum 12 and the rib cartilage 14 are corrected.
Detailed descriptions are now given of a process of correcting the sternum 12 and the rib cartilage 14 and of correlations between the process and the pectus bar 18.
Initially, immediately after correction from FIG. 2b to FIG. 2c, the sternum 12 and the rib cartilage 14 provide restoring forces to the pectus bar 18 to return to the deformed states thereof. The restoring forces are initially strong but are gradually reduced until correction is completed.
At an early stage of the above-described correction process, a surgeon inserts the pectus bar 18, which is designed to have a correction range and shape appropriate for a patient, to move the sternum 12 and the rib cartilage 14 to correction positions thereof against the strong restoring forces of the sternum 12 and the rib cartilage 14, and the two ends of the pectus bar 18 are fixed to and supported by the ribs 10 corresponding thereto to maintain the corrected state.
That is, the pectus bar 18 is provided at a position where the pectus bar 18 is flipped from FIG. 2b to FIG. 2c depending on judgment of the surgeon and, at the same time, supported by the ribs 10 against the restoring forces depending on strength of the surgeon.
The pectus bar 18 should continuously have an elastic force against the restoring forces of the sternum 12 and the rib cartilage 14.
The pectus bar 18 requires elasticity not only to respond to the restoring forces of the sternum 12 and the rib cartilage 14, but also to flexibly respond to cardiopulmonary exercise of the patient and physical forces applied from outside, together with the ribs 10.
Accordingly, at the early stage of correction, as illustrated in FIG. 2c, due to the strong restoring forces of the sternum 12 and the rib cartilage 14 in a direction indicated by an arrow B, a length-direction central part of the pectus bar 18 is slightly straightened compared to a designed shape thereof as indicated by an arrow B′ and, at the same time, two length-direction ends of the pectus bar 18 are slightly straightened away from each other as indicated by an arrow B″.
Then, for example, as illustrated in FIG. 2d, the pectus bar 18 is elastically transformed from a dashed line shape indicating the early stage of correction, to a solid line shape indicating a designed correction position.
Such transformation differs based on sizes and directions of the restoring forces of the sternum 12 and the rib cartilage 14, and a strength supported by the ribs 10.
That is, the pectus bar 18 is configured to flexibly respond to the restoring forces of the sternum and the rib cartilage 14 from the early stage of correction till the last stage of correction, and the two length-direction ends of the pectus bar 18 are variably fixed in length directions of the ribs 10 to respond to the restoring forces of the sternum 12 and the rib cartilage 14 and the elastic transformation of the pectus bar 18 based on the restoring forces.
Specifically, the position of the pectus bar 18 and, more particularly, the positions of the two ends thereof are not maintained but have clearance (tolerance) ranges in forward, backward, leftward, and rightward directions to respond to motion of the sternum 12, the rib cartilage 14, and the ribs 10 based on correction, and elastic restoration and transformation of the pectus bar 18.
The ribs 10, to which the pectus bar 18 is fixed, move from original curved positions thereof to positions indicated by the arrow B in FIG. 2c, and then slightly and continuously move during correction.
The restoring forces of the sternum 12 and the rib cartilage 14 are applied to the pectus bar 18 not only in forward, backward, leftward, and rightward directions based on the center of the pectus bar 18 but also in upward and downward directions based on the two ends of the pectus bar 18 supported by the ribs 10 as illustrated in FIG. 3.
Displacement of the pectus bar 18 due to the above-described restoring forces of the sternum 12 and the rib cartilage 14 may not only cause correction errors but also press and damage organs of a patient. A surgeon should pay close attention to avoid such problems.